The present invention relates to x-ray tubes and is particularly related to a method and apparatus for setting cathode filaments on a cathode cup for an x-ray tube.
In an operating x-ray tube, the position of the cathode filament with respect to the bottom of a slot that receives the filament is important in controlling a number of attributes of an electron beam emitted from the filament. Traditional methods for setting the operating position of the cathode filament on the cathode cup are subject to individual judgment and skill. It is difficult to make accurate and consistent placement of the cathode filament using these traditional techniques. Filaments which are not accurately placed have an undesired effect on the attributes of the electron beam.
The task of setting the filament in a cathode cup can result in scratches or other damage to the surface of the cathode cup. Such damage may cause sharp imperfections in the cathode, which can result in arcing at high voltage in an operating x-ray tube. Arcing can result in decreased x-ray tube life.
In General, setting a cathode filament begins with positioning a cathode filament at a desired location on the cathode to obtain the desired electron beam attributes. A cathode cup includes bores through the cup to receive the leads of the cathode filament. Hollow cylindrical ceramic insulators are inserted in the bores and extend through the cup. These insulators are brazed into the cathode cup. The filament is placed in the desired position by feeding filament leads through the filament is placed in the desired position by feeding filament leads through the hollow cylindrical ceramic insulators. The filament leads are then secured in the ceramic insulators and the filament is thereby supported in the cathode cup.
One presently practiced skill intensive method of positioning the filament at the desired location includes use of an optical microscope, equipped with a means for reading the location of the focal point from a reference location in units of length. Setting the desired location of the cathode filament relative to a surface of the cathode cup is initiated by inserting the cathode filament leads through the hollow cylindrical ceramic insulators. In order to position the cathode filament at the desired location, the microscope is first focused on the surface of the cathode cup and the microscope focus reading is recorded or zeroed. The microscope is then re-focused on a surface of the cathode filament and the microscope focus reading is noted. The location of the filament is adjusted to the desired location as necessary. Determining the cathode position reading with the focused microscope and the adjustment of the location of the cathode filament is conducted in an iterative process until the filament is at the desired filament setting for a specific x-ray tube application. The consistency, efficiency and accuracy of this process is influenced by the operator""s visual perception and skill. Different operators may have different visual perception and skill levels in this iterative process, which can result in inconsistent placement of cathode filaments.
Once the cathode filament is at the desired location, the filament leads are mechanically crimped within the hollow cylindrical ceramic insulators. The crimps hold the cathode filaments in the desired location during the filament flashing operations. The cathode filament is typically flashed twice, the first flashing operation is to relieve stress and the second flashing operation is to strengthen the filament. The cathode filament can twist or kink from the desired location as the mechanical stress is relieved during the first flash. If this movement changes the position of the cathode filament, resetting the filament with the microscope is difficult. After the second flash, the cathode filament leads are laser welded to the hollow cylindrical ceramic insulators. Once the filament is set, kinks in the filament are removed by physically re-aligning the kinked portions while viewing the filament under the microscope. This filament re-alignment process is also subject to the operators perception and skill. Thus, the consistency and accuracy of re-aligning the kinked filament is subject to the same individual variations as the initial setting process.
For the foregoing reasons, there is a need for a less difficult method and apparatus for efficient, accurate and consistent setting and alignment of cathode filaments.
The present invention is directed to a method and apparatus that satisfies the need to provide accurate and consistent setting of cathode filaments in x-ray tubes. A filament setting fixture apparatus in accordance with one embodiment of the present invention includes a body with a central member having a longitudinal axis. The body also includes a first end member and a second end member which are respectively located at opposite ends of the central member. Each of the end members extend away from the axis, thereby forming a recess. Each end member includes a surface generally facing the recess and an outer surface. A bore in the body is adapted to receive a retaining member for mounting the body. A cavity extends through the first end member from its outer surface to its recess facing surface. A cavity in the second end member opens toward the recess. The cavities in the first end member and second end member located opposite one another across the recess.
In accordance with another aspect of the present invention, the apparatus includes a fixture for using a mandril to set a filament on an electrode. The fixture comprises a generally c-shaped body defining a recess. A bore extends through the body for receiving a securing member to attach the fixture to the electrode in a desired location. A pair of cavities are located opposite one another across the recess formed by the c-shaped body, each cavity for retaining one end of the mandril when the mandril is extending across the recess.
In accordance with another aspect of the invention, a method is provided for positioning an electrode filament on an electrode. The method comprises the steps of passing a first end of a mandril through a first cavity in a filament setting fixture and threading the electrode filament on the mandril. The first end of the mandril is placed in a second cavity in the filament setting fixture such that the mandril extends across a recess formed by the fixture. The filament setting fixture is positioned on the electrode at a desired location and is secured to the electrode.
The present invention may include, but is not limited to, any of the following advantages such as simple, accurate and consistent positioning of electrode filaments. The present invention provides the foregoing and other features hereinafter described and particularly pointed out in the claims. The present invention may include the foregoing advantages individually or in any combination as well as with the other features described in the following description.
The following description and accompanying drawings set forth certain illustrative embodiments of the invention. It is to be appreciated that different embodiments of the invention may take form in various components and arrangements of components. These described embodiments being indicative of but a few of the various ways in which the principles of the invention may be employed. The drawings are only for the purpose of illustrating a preferred embodiment and are not to be construed as limiting the invention.